Really ?
After Obama in the 2016 election, when the Dems felt powerful, you had an actual competition between Sanders and Clinton and actual internal policy discussion. Sure, Sanders lost in the end, but that was as close as you ever got to having some left shift in the democratic party. But Trump won. 2020 Sanders he dropped out in April and the Democrats rallied early around Biden, who was particularly chosen to court undecided centrists. This time they unfortunately didn't have a proper pre-election because they had an incumbant.
Dems shouldn't have felt powerful in 2016. In the 2006 midterms is when dem fortunes started to turn for them, they remembered they were an anti-war party during 2 illegal wars. In 2008 (you might note at the end of 8 years of republican rule), Obama ran a decidedly left wing campaign to take office and won a huge smashing success, taking the house and senate. For a few months it even had a senate supermajority and dems could do literally anything they wanted.
Obama ran right so hard and so fast it cost the dems their power in 2010, which they never recovered from. After courting left wing voters to massive success, they dropped them like a rock, noted by Obama losing 5 million votes in 2012.
Then we get to 2016, saying "Bernie ran proving they were left wing" comes across quite silly, Bernie isn't even a democrat, he can run whatever the party says. He ran because the dems had created a ton of discontent among the left wing by being a bunch of right wing corpos, and Hillary was the party avatar of this. They then went on to outright cheat their primary to keep Bernie off the ballot (and argued in court that they're allowed to cheat in their own primaries). Success had bred a hard right wing streak in the dem party.
Hillary would go on to use her media connections to elevate Trump within the republican primary to make an "easy" opponent to beat, so she made it her mission to spite the left wing base while courting conservatives. She lost horribly on this strategy because it literally does not work. The dem party once again pretended to be left wing for the Trump years and in 2020 after ratfucking Bernie again, Biden made concessions during his campaign to the left and actually courted their vote. It might have been bullshit in the end, but he did win the election with more votes than even Obama got in 2008. He then made some half-hearted attempts at some of the left wing things, gave up on most of them, and his reelection chances looked grim.
Dropping out for Harris bumped the polls up, picking Walz over Shapiro (definitely seen as playing to the left over the right) kept her numbers up and she was looking great. Then she started campaigning with Cheney and we're watching her polls crash because dems are, once again, courting the right when they've been in power. The lesson from all this is that the party moves to the right when they're in power and makes overtures from the left when they're in opposition. And that electing them with overwhelming votes ensures that they blow up all their left wing promises. Exactly the opposite of what you tell us.
When the Dems feel week, they move right. When they feel strong, they allow their wings to fight.
They literally didn't allow their wings to fight, they sabotaged the primary against Bernie.